Metal cans such as steel and aluminium beverage cans are commonly manufactured in two pieces. A first part comprises a generally cylindrical container body with integral base, formed from a circular metal disk using a drawing and ironing process. A second part comprises an end having a tab or ring-pull formed therein. The can is filled, e.g. with beverage, and the end subsequently fixed to the body using a seaming process.
Can decorators are known in the art for applying decoration to the external surface of a can body. A typical decorator is used to apply decoration to the can body prior to filling of the can body and prior to seaming of the end. The prior art can decorator is a relatively complex apparatus, but is illustrated schematically in FIG. 1. On the left hand side of the illustration there is shown a can body conveying mechanism comprising a set of mandrels rotating about a common axis. Unprinted or “blank” can bodies are loaded onto the mandrels. These are then rotated into a printing zone where the can bodies are brought into contact, i.e. rolled across, pre-inked blankets mounted on a blanket wheel via respecting blanket segments. FIG. 1 illustrates a blanket wheel comprising eight blankets.
FIG. 1 also illustrates six ink stations, each comprising an ink reservoir, a printing plate (typically having an image embossed thereon), and a delivery mechanism for ensuring even application of ink from the reservoir to the printing plate. Each blanket passes through the ink stations in sequence such that a blanket leaving the final ink station has a composite (in this case, six colour) ink image formed on a printing surface thereof. This composite image is transferred to a can body in the printing zone. FIG. 2 further illustrates a 6 colour printing process, where the first five ink stations apply letters of the word “PRINT” in sequence in different colours. The final ink station (applying red ink) applies a background colour to the blanket. It will be appreciated that the word is formed in reverse on the blanket so that it appears correctly when transferred from the blanket printing surface to a can body.
In some production lines, can bodies may be pre-coated with a basecoat. This may be a white basecoat that is dried prior to the can bodies entering the can decorator (FIG. 1). The decorator then applies the colour decoration to the can body on top of the basecoat. In some cases, the basecoat may be a transparent basecoat.
The most common can decorators print different colours (i.e. corresponding to different ink stations) in non-overlapping areas of the can body. However, it is possible to print colours one on top of the other, i.e. different ink stations overprint different colours on the blankets. This is referred to as “wet-on-wet” printing.
Can decorators are described in more detail in WO 2012/148576 and U.S. Pat. No. 3,766,851.
Existing can decorators are extremely efficient at producing cans conforming to a common design. Several thousand cans per minute (e.g. 2400) can be produced by a single decorator. Even higher production rates can be achieved using so-called dual decorators which effectively use a pair of decorators aligned in parallel.
Beverage and other canning companies are extremely keen to introduce some degree of design differentiation between cans produced on single production line, i.e. using a single decorator, without having to interrupt production, e.g. to change printing plates. In particular, companies are keen to produce individual pallets including a mix of can designs. Although the required design differentiation may be relatively minor (in the context of the overall can design), e.g. designs may be differentiated by the printing of specific words at a certain position, it has proven extremely difficult to achieve this in a commercial production line.